The present invention relates in general to a photodiode structure, and more particularly to a pinned photodiode integrated with a trench isolation structure.
Recently, NPS (N-type photo sensor)/P-sub photodiodes formed under STI (shallow trench isolation) regions have been widely used due to their low dark current and high quantum efficiency. Conversely, pinned photodiode structures are used to improve dark current and blue(short wavelength) response, by forming a buried junction near the surface of the substrate, in conventional image sensors.
As is well-known to those skilled in the art, the pinned photodiode (PPD) has been widely used as an element to produce and integrate photoelectric charge generated in image sensors when detecting light from an object. These are also referred to as “buried photodiodes” as they comprise a PNP (or NPN) junction structure buried in a substrate. When compared with photodiodes of other structures, the PPD has various advantages, among them is an increased depletion depth which produces high quantum efficiency during conversion of incident photons into electric charges. That is, in a PPD comprising a PNP junction structure, the N-type region is fully depleted and the depletion region extends to both P-type regions with an increased depletion depth. Accordingly, this vertical extension of the depletion depth may increase quantum efficiency, thereby enabling an excellent light sensitivity.
The pinned photodiode structure, however, is not compatible with conventional STI technology due to the difficulty of controlling the formation of a shallow source/drain junction and the inability to cover the STI corner in the conventional process.